1. Field of the Invention
The invention relates to a process for the preparation of crude dimethyl terephthalate (crude DKT) which includes the step of distilling the crude ester in the dimethyl terephthalate/terephthalic acid process.
2. Description of the Background
Dimethyl terephthalate (DMT) and terephthalic acid (TA) are prepared industrially in numerous plants all over the world. DMT and TA are important starting compounds for the preparation of polyesters. The fields of application of the polyesters for fibers and films, inter alia, for photographic films and magnetic tapes or bottles made of poly(ethylene terephthalate), to name only a few, have long been known.
It is known that the current Witten-DMT process essentially includes the process steps,
oxidation of para-xylene (p-X) and methyl para-toluate (p-TE), generally with downstream off-gas cleaning, PA1 esterification with methanol of the reaction products from the oxidation, PA1 separation of the resulting so-called crude ester into PA1 purification of the crude DMT fraction, for example by washing, recrystallization and distillation, PA1 separating a crude ester into a plurality of fractions with a distillation column containing structured packing; PA1 wherein said column is operated at a bottom temperature of 180-260.degree. C., and PA1 said crude ester contains dimethyl terephthalate. PA1 oxidizing a mixture comprising para-xylene and methyl para-toluate, thereby forming an oxidized mixture; PA1 esterifying the oxidized mixture with methanol, thereby forming a crude ester; PA1 separating the crude ester into a plurality of fractions, including: PA1 said separating is performed with a distillation column containing structured packing; and PA1 said column is operated at an overhead temperature of 100-220.degree. C., at a pressure of 30-200 mbar, and at a bottom temperature of 180-260.degree. C.; wherein a pressure difference between the top and the bottom of said column is at most 30 mbar. PA1 a distillation column containing structured packing in contact with a crude ester which contains dimethyl terephthalate.
a) a fraction which is recycled to the oxidation, PA2 b) a crude DMT fraction which contains more than 85% by weight of DMT and PA2 c) a high-boiling residue fraction, if appropriate workup thereof, for example by methanolysis or thermolysis, and subsequent recovery of the catalyst, PA2 (a) a crude dimethyl terephthalate fraction; PA2 (b) a high-boiling residue fraction; and PA2 (c) a low-boiling recyclable fraction; wherein
("Dimethyl terephthalate", Ullmann Volume 22, 4th edition, pp. 529-533; EP 0 464 046 B1; DE-A 40 26 733). It is also possible to prepare terephthalic acid of appropriate quality from crude DMT fractions which are particularly rich in DMT or from very high purity DMT by specific hydrolysis.
A mixture of para-xylene (p-X) and methyl para-toluate (p-TE or pT ester) is generally oxidized with atmospheric oxygen in the liquid phase in the presence of a heavy metal catalyst (DE-C 20 10 137) at a temperature of about 140 to 180.degree. C. and at a pressure of about 4 to 8 bar absolute. The oxidation stage produces a reaction mixture, which predominantly comprises monomethyl terephthalate (MMT), p-toluyic acid (p-TA) and terephthalic acid (TA), dissolved or suspended in p-TE, and is esterified with methanol at a temperature of about 250 to 280.degree. C. and a pressure of 20 to 25 bar absolute. The resulting crude ester is fractionated by distillation into a p-TE fraction, a crude DMT fraction and a high-boiling catalyst-containing residue fraction. The p-TE fraction is recycled to the oxidation and the crude DMT fraction is converted to the desired product quality via subsequent purification stages. The residue fraction originating from the crude ester distillation is generally subjected to a methanolysis or thermolysis. Processes are also known for recovering and reusing the heavy metal oxidation catalyst from the high-boiling catalyst-containing residues, as are generally produced in the oxidation, esterification or crude ester distillation.
DE-A 30 11 858 discloses a process for the crude ester distillation using a three-column system and further teaches that a low-pressure-drop column is particularly suitable for the TAE distillation, e.g. a column having suitable packing or packing bodies. Such three-column systems require a high capital expenditure.
In the Witten-DMT process, the distillation units used to distill the crude ester are generally the so-called flash chamber system (FIG. 1) and the two-column system (FIG. 2). The columns of these systems are conventionally designed as tray columns. The crude ester is distilled under reduced pressure (vacuum), pressures between 20 and 200 mbar customarily being set at the tops of the columns. Relatively high bottom temperatures result from these operating conditions, which temperatures are generally in the range from 230 to 270.degree. C. It is a disadvantage that undesired side-reactions, for example the formation of so-called high-boilers, occur to an increased extent owing to high bottom temperatures. Although decreasing the pressure in the crude ester distillation could achieve a decrease of the operating temperature in the column bottoms, other disadvantages then occur; increasing the vacuum with constant plant capacity would result in higher gas volumes; columns having correspondingly larger column diameters would be required and thus also a higher capital expenditure would be required. In addition, any decrease in overhead pressures would result in lower condensation temperatures of the distillation vapors arising at the top of the columns. Customarily, the heats of condensation produced at this point are used to produce low- or medium-pressure steam. If the condensation temperature were to be markedly decreased by reducing the pressure in the column tops, the generation of steam can become uneconomic or impossible (condensation temperature &lt;100.degree. C.). This would be disadvantageous for the energy balance over the entire process.